Fist, I agree with Giovanni about the "not being there" stance. Particularly true when under a severe, strong attack. First thing is to get out of the way: move to the left, the right, backward, or anywhere you want but get out of the line (or center) of your opponent.

It may be clearer to say: keep your opponent in front of you without being in front of your opponent.

As for O'Sensei and the 70% atemi, I don't know if he said that before of after the war. Atemi were much more present in the aikido of the pre-war time. The idea was then to put your opponent out of combat (even killing him) with an atemi as soon as possible. It was totally different after the war... But that's another topic and there's already a numbers of threads about that on aikiweb.

Now, the shomen uchi. The idea isn't to make sankyo or kotae gaeshi. You do the technique most appropriate to the attack... There isn't one-fit-all attack technique... Some work better with strong attack, others just work better with less severe attacks... Attacked by a severe shomenuchi, I will favor a "let it pass you technique" like nikkyo ura, or some variation of irimi...

Now, about the grabs. An attacker try to grabs you. You don't want to be grabbed. Because you do something, your attacker choose to pull his hand because he feeled he cannot grabbed you. Your problem is solved: the attack has been countered and you haven't had to use any technique or atemi. That's quite good for me!!! Now, if an attacker really want to grab you and make a dedicated attack, there's no way he will be able to stop his attack and pull out his hand before you can apply a technique...